refactor(ui): R10 — .btn-primary recipe for the primary/action buttons

Add a paint-only .btn-primary recipe (solid accent bg + accent-fg text +
hover:opacity dim) in index.css @layer components, alongside the selection-paint
recipes (.pill/.chip/.is-current). Migrate ~38 primary/action buttons across 24
files off the hand-rolled 'bg-accent text-accent-fg hover:opacity-90', keeping
each site's geometry (padding/radius/font/gap/disabled/shadow).

Deliberately NOT migrated: accent BADGES/labels (spans), the chat message bubble,
selection toggles (tableHeader — R8 popover territory), and the round play FAB
(hover:scale, not the opacity-dim primary idiom). Three previously hover-less
primary buttons (SettingsPanel/SetupWizard/DraftSaveBar) gain the standard hover
dim for consistency.

Gate green (tsc/eslint-0/prettier/knip/vitest 79); self-E2E confirmed a migrated
button computes to accent/accent-fg and both recipe rules load.

frontend/src/components/Feed.tsx

@@ -522,7 +522,7 @@ export default function Feed({
           {isDemo ? (
             <button
               onClick={() => setFilters({ ...filters, scope: "all" })}
-              className="inline-flex items-center justify-center gap-2 px-5 py-2.5 rounded-xl font-semibold bg-accent text-accent-fg hover:opacity-90 transition"
+              className="inline-flex items-center justify-center gap-2 px-5 py-2.5 rounded-xl font-semibold btn-primary transition"
             >
               {t("feed.browseLibrary")}
             </button>
@@ -530,7 +530,7 @@ export default function Feed({
             <>
               <button
                 onClick={onOpenWizard}
-                className="inline-flex items-center justify-center gap-2 px-5 py-2.5 rounded-xl font-semibold bg-accent text-accent-fg hover:opacity-90 transition"
+                className="inline-flex items-center justify-center gap-2 px-5 py-2.5 rounded-xl font-semibold btn-primary transition"
               >
                 {t("feed.setUp")}
               </button>
@@ -711,7 +711,7 @@ export default function Feed({
           {filters.q.trim() && !isDemo && (
             <button
               onClick={() => onYtSearch(filters.q.trim())}
-              className="mt-4 inline-flex items-center gap-2 px-4 py-2 rounded-xl font-semibold bg-accent text-accent-fg hover:opacity-90 transition"
+              className="mt-4 inline-flex items-center gap-2 px-4 py-2 rounded-xl font-semibold btn-primary transition"
             >
               <Youtube className="w-4 h-4" />
               {t("feed.yt.searchFor", { query: filters.q.trim() })}